Multi-stage hydrocracker with kerosene recycle

ABSTRACT

This invention relates to a multi-stage process for hydroprocessing gas oils. Preferably, each stage possesses at least one hydrocracking zone. The second stage and any subsequent stages possess an environment having a low heteroatom content. Light products, such as naphtha, kerosene and diesel, may be recycled from fractionation (along with light products from other sources) to the second stage (or a subsequent stage) in order to produce a larger yield of lighter products, such as gas and naphtha. Pressure in the zone or zones subsequent to the initial zone is from 500 to 1000 psig lower than the pressure in the initial zone, in order to provide cost savings and minimize overcracking.

This application is a continuation-in-part of the copending applicationSer. No. 12/138,384 filed Jun. 12, 2008. Ser. No. 10/922,413 nowabandoned was filed Aug. 19, 2004, and is a continuation-in-part of10/162,774, filed Jun. 4, 2002.

FIELD OF THE INVENTION

This invention relates to a multi-stage hydrocracking process in whichlight products from the first stage, such as naphtha, kerosene anddiesel, are joined with naphtha, kerosene and diesel from other sourcesand recycled from fractionation to a second stage (or subsequent stage)hydrocracker in order to produce lighter products, such as gas andnaphtha.

BACKGROUND OF THE INVENTION

Historically, there has been little interest in cracking kerosene orother light products to even lighter products. In the United States,there is little demand for gas or other very light volatile products.Bottoms materials are usually the material recycled in two-stagehydrocracking as practiced in the United States. There is, however, ademand for products such as LPG and LNG in Asia.

Although there has been demand for very light products in some parts ofthe world, there was a belief by many experts that light products wouldnot crack in most reactors (using conventional hydrocracking catalystsas opposed to FCC catalysts) because they are in the vapor phase asopposed to the liquid phase. This belief apparently originated due tothe fact that the environment in a single-stage hydrocracker, in thepresence of H₂S and NH₃, is not conducive to cracking of light products.

The concept of recycling bottoms material back to an initialhydrocracking stage (rather than a second hydrocracking stage) is wellknown. U.S. Pat. No. 6,261,441 (Gentry et al.) discloses recycling ofbottoms material which has been hydrocracked and dewaxed back to ahydrocracker.

U.S. Pat. No. 5,447,621 (Hunter) discloses a middle distillate upgradingprocess. A middle distillate side stream of a conventional single-stagehydrocracking process is circulated to a hydrotreating stage, such as anaromatics saturation reactor and/or a catalytic dewaxing reactor inorder to effect middle distillate upgrade. The upgraded product is thenfinished in a fractionation stage side-stripper column. This inventiondiscloses passing middle distillate to a hydrotreating stage. The middledistillates are being upgraded, not cracked, as in the instantinvention.

U.S. Pat. No. 4,789,457 (Fischer et al.) discloses a process in which ahighly aromatic substantially dealkylated feedstock is processeddirectly to high octane gasoline by hydrocracking over a catalystpreferably comprising a large pore zeolite such as zeolite Y, inaddition to a hydrogenation-dehydrogenation component. The feedstock ispreferably a light cycle oil. Light cycle oil is heavier than thekerosene and naphtha cracked in the instant invention, and only onehydrocracking stage is employed in Fischer et al.

U.S. Pat. No. 3,037,930 (Mason) is directed to a two stage conversionprocess for the production of aromatic product fractions. High pressureseparators are employed following both the first and second conversionzones. There is no teaching or suggestion of the maintenance ofsubsequent zones at lower pressures, as seen in the instant invention.Mason teaches recycle of bottoms from fractionation back to the firstconversion zone and recycle of lighter materials to other conversionzones. In the instant invention bottoms are recycled to the secondconversion zone from fractionation, as are lighter fractions at times aswell. Mason does not contemplate the use of a zeolite hydrocrackingcatalyst. Mason sends bottoms back to the first conversion zone in orderto maximize aromatics. Mason employs an amorphous hydrocracking catalystand does not contemplate the use of zeolites.

The goal of Mason is to produce aromatic hydrocarbon containing productfractions.

U.S. Pat. No. 4,921,595 (Gruia) teaches passing fractionator bottomsback to the first zone or to hydrogenation or the second zone. AlthoughGruia employs a hydrocracking catalyst which comprises zeolite Y, thefeed employed is composed of polynuclear aromatics. The goal in Gruia isto prevent condensation reactions resulting from heavy feed. The goal inthe instant invention is to maximize the amount of product in thenaphtha range that is produced.

SUMMARY OF THE INVENTION

The Applicants have found that in the environment of a cleansecond-stage hydrocracker, with heteroatoms removed, light products willcrack. The examples demonstrate that the net yield of kerosene decreasedwhen recycled to the second stage on a raw feed blend basis, while thequalities of the middle distillates remained the same. Recycling thekerosene to the second stage increased the yield of 170-350° F. reformernaphtha, the product most highly valued by the customer.

The invention disclosed herein is a process for the production of lightproducts, such as gas and naphtha, by processing kerosene in a secondstage (or a subsequent stage) of a multi-stage hydrocracker. Kerosene,diesel and naphtha from other sources are included in the recycle, andsubsequent hydroprocessing stages are maintained at lower pressures thanthe initial hydroprocessing stage. This results in cost savings.

The instant invention is summarized as follows:

A method for hydroprocessing a hydrocarbon feedstock, wherein the amountof naphtha product boiling in the range from 170°-350° F. is maximized,the method employing multiple hydroprocessing zones within a singlereaction loop wherein at least one bed in each hydroprocessing zonecontains hydrocracking catalyst, and wherein the pressure in thesubsequent zone or zones is from 500 to 1000 psig lower than thepressure in the initial zone in order to provide cost savings andminimize overcracking, said method comprising the following steps:

-   -   (a) passing a hydrocarbonaceous feedstock to a first        hydroprocessing zone having one or more beds containing        hydroprocessing catalyst, said catalyst comprising a cracking        component and a hydrogenation component, wherein the cracking        component may be amorphous or zeolitic, the hydroprocessing zone        being maintained at hydroprocessing conditions, wherein the        feedstock is contacted with catalyst and hydrogen to produce a        vapor stream and a liquid stream as effluent;    -   (b) removing the vapor stream of step (a), which comprises        hydrogen, hydrogen sulfide and light hydrocarbonaceous gases        overhead;    -   (c) combining the liquid stream of step (b) with the liquid        effluent from other hydroprocessing zones;    -   (d) passing the liquid stream of step (c), which comprises        hydrocarbonaceous compounds boiling in approximately the same        range of the hydrocarbonaceous feedstock, to fractionation;    -   (e) separating the liquid stream of step (d), in fractionation,        into gas, naphtha, kerosene and diesel fractions, in addition to        the bottoms fraction;    -   (f) passing the bottoms fraction of step (e) to further        processing or recycling to one or more of the other        hydroprocessing zones of step (c);    -   (g) passing one or more of the naphtha, kerosene and diesel        fractions to further processing as products or recycling one or        more of the fractions to one or more of the other        hydroprocessing zones of step (c), the kerosene, naphtha or        diesel fractions being in combination with kerosene, naphtha or        diesel fractions from other sources, said hydroprocessing zone        or zones being maintained at hydroprocessing conditions and at        lower pressure than the first hydroprocessing zone, and        possessing an environment substantially free of H₂S, NH₃, or        other heteroatom contaminants;    -   (h) passing the effluent of step (g) to fractionation;    -   (i) recovering in fractionation an increased amount of gas and        naphtha, and a decreased amount of kerosene, in the        fractionation stage of step (h) than in the fractionation step        of step (e).

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates a two-stage hydrocracking process having thecapability for recycle of bottoms fractions, diesel fractions, kerosenefraction or naphtha fractions to the second reactor stage.

DETAILED DESCRIPTION OF THE INVENTION

Preheated oil feed in stream 1 is mixed with hydrogen in stream 2 priorto its entrance into first stage or primary hydroprocessing zone 10.This hydroprocessing zone is preferably a downflow, fixed bed reactor.This reactor contains multiple beds of hydroprocessing catalysts. Atleast one bed contains hydrocracking catalyst.

The effluent 3 of the first stage reactor, which has been hydrotreatedand partially hydrocracked, comprises a liquid stream and a vaporstream. The vapor stream 3(a) is removed overhead. It compriseshydrogen, hydrogen sulfide and light hydrocarbonaceous gases. The liquidstream 3(b) is combined with the liquid effluent from other processzones, represented by stream 4. Stream 3(b) and stream 4 are combined tocreate stream 5. Stream 5 is passed to the fractionation unit 30, whereit is separated into gas stream 6, naphtha stream 7, kerosene stream 8,diesel stream 9, and bottoms stream 14. The naphtha product mayalternately be recycled, in whole or in part, through stream 11 tostream 15, and ultimately to second stage reactor 20. Kerosene productmay alternately be recycled, in whole or in part, through stream 12 tostream 15, and ultimately to second stage reactor 20. Diesel product maybe alternately recycled, in whole or in part, through stream 13 tostream 15, and ultimately to second stage reactor 20. Bottoms materialin stream 14 may be passed to further processing (in stream 14 a) or,alternately, may be recycled in stream 14(b) to second reactor 20.Second reactor 20 represents hydroprocessing zones subsequent to thefirst hydroprocessing zone. Each of these zones possesses an environmentsubstantially free of H₂S, NH₃ or other heteroatom components.

Feeds

A wide variety of hydrocarbon feeds may be used in the instantinvention. Typical feedstocks include any heavy or synthetic oilfraction or process stream having a boiling point above 392° F. (200°C.). Such feedstocks include vacuum gas oils, heavy atmospheric gas oil,delayed coker gas oil, visbreaker gas oil demetallized oils, vacuumresidua, atmospheric residua, deasphalted oil, Fischer-Tropsch streams,and FCC streams.

Products

Although emphasis is placed on the increased production of gas andnaphtha, the process of this invention is also useful in the productionof middle distillate fractions boiling in the range of about 250-700° F.(121-371° C.). A middle distillate fraction is defined as having anapproximate boiling range from about 250° F. to 700° F. At least 75 vol%, preferably 85 vol %, of the components of the middle distillate havea normal boiling point of greater than 250° F. At least about 75 vol %,preferably 85 vol %, of the components of the middle distillate have anormal boiling point of less than 700° F. The term “middle distillate”includes the diesel, jet fuel and kerosene boiling range fractions. Thekerosene or jet fuel boiling point range refers to the range between280° F. and 525° F. (38-274° C.). The term “diesel boiling range” refersto hydrocarbons boiling in the range from 250° F. to 700° F. (121-371°C.).

Gasoline and naphtha production is emphasized in the process of thisinvention. Gasoline or naphtha normally boils in the range below 400° F.(204° C.), or C₁₀—. Boiling ranges of various product fractionsrecovered in any particular refinery will vary with such factors as thecharacteristics of the crude oil source, local refinery markets, andproduct prices.

Heavy hydrotreated gas oil, another product of this invention, usuallyboils in the range from 550° F. to 700° F.

Conditions

Hydroprocessing conditions is a general term which refers primarily inthis application to hydrocracking or hydrotreating, preferablyhydrocracking. The first stage reactor, as depicted in FIG. 1, is apartial conversion hydrocracker.

Typical hydrocracking conditions include a reaction temperature of from400° F.-950° F. (204° C.-510° C.), preferably 650° F.-850° F. (343°C.-454° C.). Reaction pressure ranges from 500 to 5000 psig (3.5-4.5MPa), preferably 1500-3500 psig (10.4-24.2 MPa). LHSV ranges from 0.1 to15 hr⁻¹ (v/v), preferably 0.25-2.5 hr⁻¹. Hydrogen consumption rangesfrom 500 to 2500 SCF per barrel of liquid hydrocarbon feed (89.1-445m³H₂/m³ feed). Reactors subsequent to the first hydroprocessing reactorare operated at a pressure from 500 to 1000 psig lower than the firstreactor.

Catalyst

Each hydroprocessing zone may contain only one catalyst, or severalcatalysts in combination.

The hydrocracking catalyst generally comprises a cracking component, ahydrogenation component, and a binder. Such catalysts are well known inthe art. The cracking component may include an amorphous silica/aluminaphase and/or a zeolite, such as a Y-type or USY zeolite. Catalystshaving high cracking activity often employ REX, REY and USY zeolites.The most suitable zeolites of this invention possess a SiO₂/Al₂O₃ ratioof from 3 through 160, preferably of from 5 through 20. The unit cellsize of the zeolites of this invention is generally in the range from24.25 through 24.60, and preferably in the range from 24.30 through24.55. The binder is generally silica or alumina. The hydrogenationcomponent will be a Group VI, Group VII, or Group VIII metal or oxidesor sulfides thereof, preferably one or more of iron, chromium,molybdenum, tungsten, cobalt, or nickel, or the sulfides or oxidesthereof. If present in the catalyst, these hydrogenation componentsgenerally make up from about 5% to about 40% by weight of the catalyst.Alternatively, noble metals, especially platinum and/or palladium, maybe present as the hydrogenation component, either alone or incombination with the base metal hydrogenation components iron, chromiummolybdenum, tungsten, cobalt, or nickel. If present, the platinum groupmetals will generally make up from about 0.1% to about 2% by weight ofthe catalyst.

Hydrotreating catalyst usually is designed to remove sulfur and nitrogenand provide a degree of aromatic saturation. It will typically be acomposite of a Group VI metal or compound thereof, and a Group VIIImetal or compound thereof supported on a porous refractory base such asalumina. Examples of hydrotreating catalysts are alumina supportedcobalt-molybdenum, nickel sulfide, nickel-tungsten, cobalt-tungsten andnickel-molybdenum. Typically, such hydrotreating catalysts arepresulfided.

Catalyst selection is dictated by process needs and productspecifications. In particular, a noble catalyst may be used in thesecond stage when there is a low amount of H₂S present.

The Examples below demonstrate the relative effectiveness of recyclingkerosene to produce lighter products of high quality, as opposed to notrecycling kerosene.

EXAMPLE

The “recycle” of kerosene was simulated by passing kerosene from thefirst hydrocracking stage over the catalyst in the second hydrocrackingstage. The first stage kerosene possessed a smoke point of 14 mm and 25LV % aromatics. Net yields from the runs where kerosene was “recycled”have been calculated by deducting the supplemental kerosene feed fromthe gross, measured kerosene yield (gross weight of keroseneproduct-weight of kerosene “recycled”=net yield of kerosene product).

In kerosene recycle mode, a base metal zeolite hydrocracking catalystcracked a substantial fraction of the kerosene to naphtha and gas (seeTables 1 and 2). The net yield of kerosene decreased on a raw feed blendbasis and the qualities of the middle distillates remained the same.Recycling the kerosene to the second stage did increase the yield of170-350° F. reformer naphtha, a product in most demand by the customer.

TABLE 1 Two-Stage Hydrocracking of Vacuum Gas Oil/Hydrocracking Gas Oil/Light Cycle Oil Feed Blend Using Hydrocracking Catalyst Run Hours600-624 Reactor 1 Temp, ° F. 725 Reactor 2 Temp, ° F. 669 Overall LHSV,hr⁻¹ 1.00 Per Pass Conversion 58 Total Pressure, PSIG 2297 No Loss Prod.Yields Wt. % Vol. % C₁ 0.13 C₂ 0.18 C₃ 0.56 iC₄ 0.94 1.62 nC₄ 0.63 1.06C₅-170° F. 3.43 5.04 170-350° F. 13.04 16.48 350-550° F. 29.99 33.44550-RCP 15.57 16.92 Recycle Bleed 34.84 38.17 Recycle Cut Point, ° F.656 Total C₄− 2.44 Total C₅+ 96.87 110.04 Closure 99.6 // FractionatorBottoms Nitrogen, ppm 24.5

TABLE 2 Two-Stage Hydrocracking of Vacuum Gas Oil/Hydrocracked Gas Oil/Light Cycle Oil Feed Blend Using Hydrocracking Catalyst, with “KeroseneRecycle” Hours 816-840 Reactor 1 Temperature, ° F. 725 Reactor 2Temperature, ° F. 691 LHSV, 1/Hr 1.00 Per Pass Conversion, % 60 TotalPressure, psig 2294 No Loss Product Yields Wt. % Vol % C₁ 0.13 C₂ 0.20C₃ 0.80 iC₄ 1.80 nC₄ 0.99 C₅-170° F. 6.4 9.5 170-350° F. 18.0 22.8350-550° F. 24.0 26.8 550-650° F. 15.3 16.6 650° F.+ 32.4 35.3 Recyclecut point 650° F. Total C₅+ 96.1 111.0 Total C₄− 3.72 Chemical H₂Consumption, SCF/B 2080 Closure, % 99.7 Fractionator Bottoms Nitrogen,ppm 28

1. A method for hydroprocessing a hydrocarbon feedstock, wherein theamount of naphtha product boiling in the range from 170°-350° F. ismaximized, the method employing multiple hydroprocessing zones within asingle reaction loop wherein at least one bed in each hydroprocessingzone contains hydrocracking catalyst, wherein the pressure in thesubsequent zone or zones is from 500 to 1000 psig lower than thepressure in the initial zone in order to provide cost savings andminimize overcracking, said method comprising the following steps: (a)passing a hydrocarbonaceous feedstock to a first hydroprocessing zonehaving one or more beds containing hydroprocessing catalyst, saidcatalyst comprising a cracking component and a hydrogenation component,wherein the cracking component is selected from the group consisting ofY, USY, REX and REY zeolites, the hydroprocessing zone being maintainedat hydroprocessing conditions, wherein the feedstock is contacted withcatalyst and hydrogen to produce a vapor stream and a liquid stream aseffluent; (b) removing the vapor stream of step (a), which compriseshydrogen, hydrogen sulfide and light hydrocarbonaceous gases overhead;(c) combining the liquid stream of step (b) with the liquid effluentfrom other hydroprocessing zones; (d) passing the liquid stream of step(c), which comprises hydrocarbonaceous compounds boiling inapproximately the same range of the hydrocarbonaceous feedstock, tofractionation; (e) separating the liquid stream of step (d), infractionation, into gas, naphtha, kerosene and diesel fractions, inaddition to the bottoms fraction; (f) passing the bottoms fraction ofstep (e) to further processing or recycling to one or more of the otherhydroprocessing zones of step (c); (g) passing one or more of thenaphtha, kerosene and diesel fractions to further processing as productsor recycling one or more of the fractions to one or more of the otherhydroprocessing zones of step (c), the kerosene, naphtha or dieselfractions being in combination with kerosene, naphtha or dieselfractions from other sources, said hydroprocessing zone or zones beingmaintained at hydroprocessing conditions and at a pressure that is 500to 1000 psig lower than the initial hydroprocessing zone, and possessingan environment substantially free of H₂S, NH₃, or other heteroatomcontaminants; (h) passing the effluent of step (g) to fractionation; (i)recovering in fractionation an increased amount of gas and naphtha, anda decreased amount of kerosene, in the fractionation stage of step (h)than in the fractionation step of step (e).
 2. The process of claim 1,wherein at least one bed in each hydroprocessing zone containshydrocracking catalyst.
 3. The process of claim 1, wherein thehydroprocessing conditions of claim 1, step (a), and claim 1, step (g),comprise a reaction temperature of from 400° F.-950° F. (204° C.-510°C.), a reaction pressure in the range from 500 to 5000 psig (3.5-34.5MPa), an LHSV in the range from 0.1 to 15 hr⁻¹ (v/v), and hydrogenconsumption in the range from 500 to 2500 scf per barrel of liquidhydrocarbon feed (89.1-445 m³H₂/m³ feed).
 4. The process of claim 3,wherein the hydroprocessing conditions of claim 1, step (a), and claim1, step (g), preferably comprise a temperature in the range from 650°F.-850° F. (343° C.-454° C.), reaction pressure in the range from1500-3500 psig (10.4-24.2 MPa), LHSV in the range from 0.25 to 2.5 hr⁻¹,and hydrogen consumption in the range from 500 to 2500 scf per barrel ofliquid hydrocarbon feed (89.1-445 m³H₂/m³ feed).
 5. The process of claim1, wherein the feed to claim 1, step (a), comprises hydrocarbons boilingabove 392° F.(200° C.).
 6. The process of claim 5, wherein the feed isselected from the group consisting of vacuum gas oil, heavy atmosphericgas oil, delayed coker gas oil, visbreaker gas oil, demetallized oils,FCC light cycle oil, vacuum residua deasphalted oil, Fischer-Tropschstreams, and FCC streams.
 7. The process of claim 1, wherein thehydrogenation component is selected from Group VI, Group VII or GroupVIII metals.
 8. The process of claim 7, wherein the hydrogenationcomponent is selected from the group consisting of Ni, Mo, W, Pt and Pdor combinations thereof.
 9. The process of claim 7, wherein the GroupVI, Group VII or Group VIII metals may exist as either sulfides oroxides.
 10. The process of claim 7, wherein the hydrogenation componentcomprises 5 to 40 wt % of the catalyst.
 11. The process of claim 8,wherein noble metals comprise from about 0.1 wt % to about 2 wt % of thecatalyst.
 12. The process of claim 1, wherein the zeolitic component hasa SiO₂/Al₂O₃ molecular ratio in the range from 3 through
 60. 13. Theprocess of claim 12, wherein the SiO₂/Al₂O₃ molecular ratio is in therange from 5 through
 20. 14. The process of claim 1, wherein thezeolitic component has a unit cell size in the range from 24.25 through24.60.
 15. The process of claim 14, wherein the zeolitic component has aunit cell size in the range from 24.30 through 24.55.